Plant hormone

Plant hormones (also known as plant growth regulators (PGRs) and phytohormones) are chemicals that regulate plant growth. According to a standard animal definition, hormones are signal molecules produced at specific locations, that occur in very low concentrations, and cause altered processes in targeted cells at other locations. Plant hormones on the other hand, are distinct from animal hormones, since they are often not transported to other parts of the plant and production is not limited to specific locations. Plants lack tissues or organs specifically for the production of hormones; unlike animals, plants lack glands that produce and secrete hormones that are then circulated around the body. Plant hormones shape the plant, effecting seed growth, time of flowering, the sex of flowers, senescence of leaves and fruits, they affect which tissues grow upward and which grow downward, leaf formation and stem growth, fruit development and ripening, plant longevity and even plant death. Hormones are vital to plant growth and lacking them, plants would be mostly a mass of undifferentiated cells.

Characteristics

The word hormone is derived from Greek and means 'set in motion'. They are naturally produced within plants, though very similar chemicals are produced by fungi and bacteria that can effect plant growth.[1] Also a large number of related chemical compounds synthesized by humans that function as hormones are called plant growth regulators, or PGRs for short. At the beginning of the study of plant hormones, "phytohormone" was the commonly-used term, but its use is less widely applied now.

Plant hormones are not nutrients, but chemicals that in small amounts promote and influence the growth, development and differentiation of cells and tissues. The biosynthesis of plant hormones within plant tissues is often diffuse and not always localized, and, unlike animals, which have two circulatory systems (lymphatic and cardiovascular) powered by one or more hearts that move fluids around the body, plants utilize simple chemical hormones that move more easily through the plants tissues. They are often produced very locally within the plant body, some plant cells even produce hormones that effect the same cell. Hormones are transported within the plant by utilizing four types of movements; cytoplasmic streaming within cells, slow diffusion of ions and molecules between cells, sieve tubes that move sugars from the leaves to the roots and flowers, plus xylem that moves water and mineral solutes from the roots to the foliage. Plants need hormones at very specific times during plant growth and at specific locations, they also need to disengage the effects hormones have when they are no longer needed. The production of hormones occurs very often at sites of active growth within the meristems, before cells have fully differentiated out. After production they are sometimes moved to other parts of the plant were they cause an immediate effect or they can be stored in cells to be released later. Plants use different pathways to regulate internal hormone quantities and moderate there effects; they can regulate the amount of chemicals used to bio-synthesize hormones, they can inactivate or canabilize already-formed hormones by conjugating them with carbohydrates, amino acids or peptides, or plants can break down hormones - effectively destroying them or store them in cells. Plants also move hormones around the plant, diluting their concentrations. Not all plant cells respond to hormones, but cells that do are programmed to respond at specific points in their growth cycle; so the greatest effects occur at a specific stage in the cell's life, with diminished effects occurring before or after this period.

The concentration of hormones required for plant responses are very low (10-6 to 10-5mol/L), due to these low levels it has been very difficult to study plant hormones and only since the late 1970s have scientists been able to start piecing together their effects and relationships to plant physiology. Much of the early work on plant hormones involved studying plants that were genetically deficient in one or involved the use of tissue cultured plants grown in vitro that were subjected to differing ratios of hormones and the resultant growth compared. The earliest scientific observation and study dates to the 1880s, the determination and observation of plant hormones and there identification was spread-out over the next 70 years.

Classes of plant hormones

It is generally accepted that there are five major classes of plant hormones, some of which are made up of many different chemicals that can vary in structure from one plant to the next. The chemicals are each grouped together into one of these classes based on their structural similarities and on their effects on plant physiology. Other plant growth regulators that are not easily grouped into these classes exist naturally, including chemicals that inhibit plant growth or interrupt the physiological processes within plants. Each class has positive as well as inhibitory functions, and they most often work in tandem with each other, with varying ratios of one or more interplaying to affect growth regulation.[2]

The five major classes are:

Abscisic acid

Abscisic acid also called ABA, was discovered and researched under two different names before its chemical properties were fully known, it was called dormin and abscicin II. Once it was determined that the two latter named compounds were the same, it was named abscisic acid. The name "abscisic acid" was given because it was found in high concentrations in newly-abscissed or freshly-fallen leaves.

This class of PGR is composed of one chemical compound normally produced in the leaves of plants, originating from chloroplasts, especially when plants are under stress. In general, it acts as an inhibitory chemical compound that effects bud growth, seed and bud dormancy. It mediates changes within the apical meristem causing bud dormancy and the alteration of the last set of leaves into protective bud covers. Since it was found in freshly-adscissed leaves, it was thought to play a role in the processes of natural leaf drop but further research has disproven this. In plant species from temperate parts of the world it plays a role in leaf and seed dormancy by inhibiting growth, but, as it is dissipated from seeds or buds, growth begins. In other plants, as ABA levels decrease, growth then commences as gibberellin levels increase. Without ABA, buds and seeds would start to grow during warm periods in winter and be killed when it froze again. Since ABA dissipates slowly from the tissues and its effects take time to be offset by other plant hormones, there is a delay in physiological pathways that provide some protection from premature growth. It accumulates within seeds during fruit maturation, preventing seed germination within the fruit, or seed germination before winter. Abscisic acid's effects are degraded within plant tissues, during cold temperatures or by its removal by water washing in out of the tissues, releasing the seeds and buds from dormancy.[3]

In plants water stressed, ABA plays a role in closing the stomata. Soon after plants are water stressed and the roots are deficient in water, a signal moves up to the leaves causing the formation of ABA precursors, these precursors move to the roots which release ABA that is translocated to the foliage through the vascular system, [4] which regulates the potassium or sodium uptake within the guard cells, which then loses turgidity, closing the stomata.[5][6]
ABA exists in all parts of the plant and its concentration within any tissue seems to mediate its effects and function as a hormone, its degradation or more properly catabolism within the plant affects metabolic reactions and cellular growth and production of other hormones.[7] Plants start life as a seed with high ABA levels, just before the seed germinates ABA levels decrease; during germination and early growth of the seedling, ABA levels decrease even more. As plants begin to produce shoots with fully functional leaves - ABA levels begin to increase, slowing down cellular growth in more "mature" areas of the plant. Stress from water or predation effects ABA production and catabolism rates which mediate another cascade of effects triggering specific responses from targeted cells. Scientists are still piecing together the complex interactions and effects of this and other phytohormones.

Auxins

Auxins are compounds that positively influence cell enlargement, bud formation and root initiation. They also promote the production of other hormones and in conjunction with cytokinins, they control the growth of stems, roots, flowers and fruits.[8] Auxins were the first class of growth regulators discovered.[9] They affect cell elongation by altering cell wall plasticity. Auxins decrease in light and increase where its dark. They stimulate cambium cells to divide and in stems cause secondary xylem to differentiate. Auxins act to inhibit the growth of buds lower down the stems, affecting a process called apical dominance, and also promote lateral and adventitious root development and growth. Auxins promote flower initiation, converting stems into flowers. When auxins are no longer produced by the growing point of a plant, this initiates leaf abscission. Seeds produce auxins, that regulate specific protein synthesis,[10] as they develop within the flower after pollination, causing the flower to develop a fruit to contain the developing seeds. Auxins are toxic to plants in large concentrations; they are most toxic to dicots and less so to monocots. Because of this property, synthetic auxin herbicides including 2,4-D and 2,4,5-T have been developed and used for weed control. Auxins, especially 1-Naphthaleneacetic acid (NAA) and Indole-3-butyric acid (IBA), are also commonly applied to stimulate root growth when taking cuttings of plants. The most common auxin found in plants is indoleacetic acid or IAA.

Cytokinins

Cytokinins or CKs are a group of chemicals that influence cell division and shoot formation. They were called kinins in the past when the first cytokinins were isolated from yeast cells. They also help delay senescence or the aging of tissues, are responsible for mediating auxin transport throughout the plant, and affect internodal length and leaf growth. They have a highly-synergistic effect in concert with auxins and the ratios of these two groups of plant hormones affect most major growth periods during a plant's lifetime. Cytokinins counter the apical dominance induced by auxins; they in conjunction with ethylene promote abscission of leaves, flower parts and fruits.[11]

Ethylene

Ethylene is a gas that forms from the breakdown of methionine, which is in all cells. Ethylene has very limited solubility in water and does not accumulate within the cell but diffuses out of the cell and escapes out of the plant. Its effectiveness as a plant hormone is dependent on its rate of production versus its rate of escaping into the atmosphere. Ethylene is produced at a faster rate in rapidly-growing and -dividing cells, especially in darkness. New growth and newly-germinated seedlings produce more ethylene than can escape the plant, which leads to elevated amounts of ethylene, inhibiting leaf expansion. As the new shoot is exposed to light, reactions by photochrome in the plant's cells produce a signal for ethylene production to decrease, allowing leaf expansion. Ethylene affects cell growth and cell shape; when a growing shoot hits an obstacle while underground, ethylene production greatly increases, preventing cell elongation and causing the stem to swell. The resulting thicker stem can exert more pressure against the object impeding its path to the surface. If the shoot does not reach the surface and the ethylene stimulus becomes prolonged, it affects the stems natural geotropic response, which is to grow upright, allowing it to grow around an object. Studies seem to indicate that ethylene affects stem diameter and height: When stems of trees are subjected to wind, causing lateral stress, greater ethylene production occurs, resulting in thicker, more sturdy tree trunks and branches. Ethylene affects fruit-ripening: Normally, when the seeds are mature, ethylene production increases and builds-up within the fruit, resulting in a climacteric event just before seed dispersal. The nuclear protein ETHYLENE INSENSITIVE2 (EIN2) is regulated by ethylene production, and, in turn, regulates other hormones including ABA and stress hormones.[12]

Gibberellins

Gibberellins or GAs include a large range of chemicals that are produced naturally within plants and by fungi. They were first discovered when Japanese researches noticed a chemical produced by a fungus called Gibberella fujikuroi that produced abnormal growth in rice plants. They play a major role in seed germination, affecting enzyme production that mobilizes food production that new cells need for growth. This is done by modulating chromosomal transcription. In seedlings a layer of cells called the aleurone layer wraps around the endosperm tissue: During seed germination, the seedling produces GA that is transported to the aleurone layer, which responds by producing enzymes that break down stored food reserves within the endosperm, which are utilized by the growing seedling. GAs produce bolting of rosette-forming plants, increasing internodal length. They promote flowering, cellular division, and in seeds growth after germination. Gibberellins also reverse the inhibition of shoot growth and dormancy induced by ABA.[13]

Other known hormones

Other identified plant growth regulators include:

brassinolides are plant steroids chemically similar to animal steroid hormones. First isolated from pollen of the mustard family and extensively studied in Arabidopsis. They promote cell elongation and cell division, differentiation of xylem tissues, and inhibit leaf abscission.[14] Plants found deficient in brassinolides suffer from dwarfism.

salicylic acid in some plants activates genes that assist in the defense against pathogenic invaders.

jasmonates are produced from fatty acids and seem to promote the production of defense proteins that are used to fend off invading organisms. They are believed to also have a role in seed germination, the storage of protein in seeds and seem to effect root growth.

signalling peptides

Systemin

Potential medical applications

Plant stress hormones activate cellular responses, including cell death, to diverse stress situations in plants. Researchers have found that some plant stress hormones share the ability to adversely affect human cancer cells [1]. For example, sodium salicylate has been found to suppress proliferation of lymphoblastic leukemia, prostate, breast, and melanoma human cancer cells. Jasmonic acid, a plant stress hormone that belongs to the jasmonate family, induced death in lymphoblastic leukemia cells. Methyl jasmonate has been found to induce cell death in a number of cancer cell lines.

Hormones and plant propagation

Synthetic plant hormones or PGRs are commonly used in a number of different techniques involving plant propagation from cuttings, grafting, micropropagation, and tissue culture.

The propagation of plants by cuttings of fully-developed leaves, stems, or roots is performed by gardeners utilizing auxin as a rooting compound applied to the cut surface; the auxins are taken into the plant and promote root initiation. In grafting, auxin promotes callus tissue formation, which joins the surfaces of the graft together. In micropropagation, different PGRs are used to promote multiplication and then rooting of new plantlets. In the tissue-culturing of plant cells, PGRs are used to produce callus growth, multiplication, and rooting.

Seed dormancy

Plant hormones affect seed germination and dormancy by affecting different parts of the seed.

Embryo dormancy is characterized by a high ABA/GA ratio, whereas the seed has a high ABA sensitivity and low GA sensitivity. To release the seed from this type of dormancy and initiate seed germination, an alteration in hormone biosynthesis and degradation towards a low ABA/GA ratio, along with a decrease in ABA sensitivity and an increase in GA sensitivity, needs to occur. ABA controls embryo dormancy, and GA embryo germination.
Seed coat dormancy involves the mechanical restriction of the seed coat, along with low embryo growth potential, effectively produces seed dormancy. GA releases this dormancy by increasing the embryo growth potential, and/or weakening the seed coat so the radical of the seedling can break through the seed coat.
Two different types of seed coats that are influenced by hormones exist; one is composed of living cells, whereas the other is composed of dead cells. Testa dormancy can be influenced during the formation of the seed coat. ABA affects the testa growth characteristics, including thickness, and GA-mediated embryo growth potential. These conditions and effects occur during the formation of the seed, often in response to environmental conditions. Hormones also mediate endosperm dormancy: Endosperm in most seeds is composed of living tissue that can actively respond to hormones generated by the embryo. The endosperm often acts as a barrier to seed germination, playing a part in seed coat dormancy, or a part in the germination process. Since the living cells respond to and affect the ABA/GA ratio and sensitivity, GA increasing the embryo growth potential and can promote endosperm weakening. GA affects both ABA-independent and ABA-inhibiting processes within the endosperm.[15]